Pollen and Pistillate Flowerabscission

The objective of this year's work was to confirm the role of pollen in pistillate flower abscission (PFA) of Serr walnut. Pistillate flower abscission was evaluated in individual trees along transects, beginning close to po11enizers and extending up to 640 feet away, in 12 orchards. It was also evaluated in controlled pollinations of bagged and unbagged Serr flowers with varying levels of Serr and Tehama pollen. In both cases the number of pollen grains present on the flower stigma surfaces was counted in representative samples. In the orchards there was a significant decrease in PFA as distance from the po1lenizer increased. There was also a significant decrease in number of pollen grains on the stigmas with distance and a strong correlation between number of pollen grains and PFA. In the controlled pollinations, flowers which were not exposed to pollen did not abscise at the PFA stage~ whereas flowers exposed to pollen had abscission ranging from 22% (Tehama pollen, low dose) to 77% (Serr pollen, low dose). Small flowers that were expected to abscise had significantly more pollen on them than normal healthy flowers. These results confirm earlier observations that pollen load contributes to pistillate flower abscission in walnuts. Future work will be aimed at understanding the mechanism behind the relationship and at providing management and breeding options to avoid PFA in the future.